Although in the mammalian virion reverse transcriptase (RT) can replicate the genome, after purification from the viral core or gag proteins it loses this ability. This suggests that the gag proteins are important in insuring proper functioning of RT. Therefore, we pursued the hypothesis that in the intact virion there exists a DNA synthesizing apparatus consisting of RT and gag proteins organized in a specific manner. We have succeeded in uncovering certain gag protein-RT interactions utilizing both purified proteins and virion cores from Rauscher leukemia virus (RLV) and hope that soon for the first time we may be able to synthesize infectious DNA from highly purified viral components.